An experimental drug can improve sociability in patients with fragile X syndrome and may be helpful as a treatment for autism, according to the authors of a new study. Fragile X is a rare genetic disorder that affects about 1 in 4,000 boys and 1 in 8,000 girls, according to the National Institutes of Health. It usually results in mental retardation and — in about half of cases — some form of autism. In fragile X, which accounts for 2% of autism cases, a mutation in a gene on the X chromosome turns off production of a regulatory protein known as FMRP. That leads to out-of-control activation of the brain chemical glutamate, which plays a key role in learning and memory, potentially explaining social anxiety and other symptoms of the disorder.